Dark current based on field-emission current is considered as a factor causing the vacuum electrical breakdown between the multi-aperture acceleration grids in the JT-60 negative ion source. In this paper, we focused on field enhancement factor which is a key parameter of field-emission from the electrode. Vacuum breakdown tests for small sized electrodes simulating the multi-aperture acceleration grids of the negative ion source was conducted. We obtained field enhancement factor and breakdown field for multi-aperture electrodes, and we investigated the dependence of each parameter on the number of apertures. The results revealed that the increase of the average field enhancement factor after the end of conditioning due to the increase of the number of apertures decreases dielectric strength of the multi-aperture electrode.

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We have been studying a cryogenic electrical insulation system with PPLP (Polypropylene laminated paper)^® for its application to high-temperature superconducting cable. PPLP^®, which has the three-laminated structure of a kraft paper (KP), polypropylene (PP) film and a KP, has been reported to have an excellent insulation performance in liquid nitrogen (LN₂). However, questions still remain in dielectric breakdown mechanism of PPLP^® due to its laminated structure. In this paper, it is investigated the influence of prestressing on the impulse breakdown characteristic of KP and PP film to understand the basic breakdown characteristic of PPLP^® in LN₂ and we discussed the charge injection model into KP and PP film. As the result, the breakdown strength of KP and PP film under hetero polarity DC prestressing field application was not decreased compared with normal impulse breakdown strength, which was different from the general DC prestressing effects. Furthermore, it is particularly interesting that the breakdown strength of KP was increased under the positive DC prestressing field with negative impulse field in spite of hetero polarity DC prestressing field application. This result could be explained based on the charge injection model, in which negative charge may be easily injected inside KP through cellulose fiber interface.

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CF₃I gas is one of the alternatives for SF₆ which has been applied for electric power equipment. CF₃I has some merit against SF₆. For example, its insulation performance is equivalent or superior to SF₆ in uniform field and its Global Warming Potential (GWP) is quite low as same as CO₂. On the other hand, it has low sparkover voltage in non-uniform field. This paper reviews the discharge characteristics of CF₃I and CF₃I/N₂, CF₃I/CO₂ and CF₃I/SF₆ mixtures and tries to find the pragmatic gas as an insulation medium with a simple ranking method where saturated gas pressure and GWP are taken into account.

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This journal article shows the effect of cooling with liquid nitrogen on a gallium nitride (GaN) film that is exposed in argon plasma. We evaluated the damage induced by plasma on a n-GaN film (2µm) with photoluminescence (PL). Our preliminary result shows that the PL intensity less than 450nm stayed almost constant only when the film was cooled down with liquid nitrogen. This can be an indication that the damages induced by plasma can be avoided for the wavelength less than 450nm by cooling the film.

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We propose here an 8-channel linear array system for imaging electrical and ultrasonic properties in human tissues. An array of piezoelectric ceramic transducers (PZTs) is used for ultrasound tissue imaging whereas the electrode in the PZTs is used for electrical impedance imaging. The capability of the system to detect a small volume with distinct electrical conductivity in tissue was evaluated using a tissue model system. The present electrical impedance imaging alone failed to accurately locate the small object with a different conductivity in the model system. However, when the reconstruction of electrical impedance was combined with ultrasonic determination of the position of the object of interest, our method successfully assessed the conductivity of the target object. Thus, synergetic use of ultrasound data in electrical impedance imaging significantly improves the accuracy of tissue impedance imaging.

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This paper describes a correlation analysis of the atmospheric state change and lightning discharges in the summer (from June to Aug. 2011) and winter (from Dec. 2011 to Jan. 2012) seasons in Japan. In evaluating the number of lightning discharges, we calculated SSI and K-indices, which are used to predict thunderstorms. As a result, we found that there are close relationships between two indices and lightning discharge activities. However, the results suggest that the conditions for calculating SSI need to be modified in evaluating winter lightning discharge activity.

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